In order to fabricate semiconductor devices, such as semiconductor wafers used in integrated circuits, a number of various processes are used to form circuit features and conductive paths on the wafer substrate. One such process involves photolithography wherein a photosensitive material referred to as a photoresist is applied to the face of the substrate, and is subsequently exposed to light through a mask possessing features related to the circuit paths to be formed. Following such exposure, the un-exposed portions of the photoresist, i.e. those covered by the mask, are removed to form the desired conductive paths. The photoresist material is removed by subjecting the exposed photoresist coating to a specialized acid stripper, typically in the form of a bath. The photoresist layer is actually a mixture of a photoresist material combined with a chosen polymer and chemically held together by an organic bond.
As the photoresist stripper bath is used to process successive batches of wafers, the stripped, photoresist material builds up in the stripper bath, thus diluting the concentration of the photoresist stripper acid, and reducing its effectiveness accordingly. In order to solve this problem, the photoresist stripper acid is re-circulated through a filter which filters out the unwanted photoresist materials, thus maintaining the stripper acid above a desired concentration threshold.
In spite of the use of the filtering system mentioned above, the photoresist stripper acid concentration level sometime falls below the desired threshold level for any of a number of reasons, including reduction in the effectiveness of the filter due to residue buildup or other factors, or even complete failure of the filter. In the past, there has been no means by which an operator could be alerted to the fact that the stripper acid concentration had fallen below a threshold level. In other words, there has not previously been a means for monitoring or detecting, on a real time basis, the stripper acid concentration level. As a result, one or more batches of the wafers are not properly processed, thus resulting in defective wafers which in turn reduce throughput and yield in the fabrication process. Accordingly, there is a clear need in the art for a method and apparatus for controlling the quality of a photoresist stripper bath which overcomes each of deficiencies of the prior art mentioned above.